Titanium oxide powder has been used as a white pigment for many years. More recently, titanium oxide powder is widely used as a UV shielding material for cosmetics and the like, a material for forming a photocatalyst, a capacitor, or a thermistor, and a sintered material used for electronic materials such as a raw material of barium titanate. Due to possession of a large refractive index in a wavelength region near visible light, the titanium oxide absorbs almost no light in the visible-light region. For these reasons, titanium oxide powder is recently used as a material that requires UV shielding properties such as a cosmetic composition, drug, or coating material and as an antireflection film for a display part of a liquid crystal display and a plastic lens. The antireflection film typically comprises a layer formed by alternately laminating a layer of a resin with a low refractive index, such as a fluororesin or silicon-containing resin, and a high refractive index layer. Titanium oxide is used as a material for the high refractive index layer of the antireflection film. In addition, in plasma displays for which demand is recently increasing, a glass material used for substrate partitions is covered with titanium oxide in order to promote brightness and improve reflectance or is caused to contain rutile-type titanium oxide powder in order to improve the refractive index.
The rutile-type titanium oxide is more excellent than anatase-type titanium oxide in optical characteristics, such as UV shielding characteristics and high refractive index, and electrical properties such as high dielectric properties.
Titanium oxide is excited when irradiated with light having energy not less than its band gap and produces electrons in the conduction band and positive holes in the valence band. Development for application of a photocatalyst utilizing a reduction power of the electrons and an oxidation power of positive holes is actively undertaken. There are various applications of the titanium oxide photocatalyst. A number of application developments such as hydrogen production by decomposition of water, exhaust gas treatment, air cleaning, deodorization, sterilization, antibacterial treatment, waste water treatment, stain-proofing of illumination equipment, and the like are ongoing.
The anatase-type titanium oxide is used as a photocatalyst material due to the high catalytic activity.
As a method for obtaining anatase-type titanium oxide, Japanese Patent Application Laid-open No. 8-333117 discloses a method for producing anatase-type titanium oxide particles having a uniform particle size and a large specific surface area. The method comprises preparing an aqueous solution of titanyl sulfate containing titanyl sulfate in an amount of 5.0-100 g/l (as TiO2) and an excess amount of sulfuric acid of 1.0-3.0 times of Ti (as mol), adding urea in an amount equimolar or more to the total sulfate group, heating the mixture at a temperature from 85° C. to boiling point to collect deposited meta-titanic acid particles, and baking the meta-titanic acid particles at 650-850° C.
Japanese Patent Application Laid-open No. 2001-287997 discloses a method of oxidizing titanium tetrachloride in a gas phase oxidation reaction by causing 1 l titanium tetrachloride gas to come in contact with 1-30 l of oxygen (gas volume being indicated in a standard state gas volume) at 700-850° C. and to produce titanium oxide particles and heat-treating the resulting titanium oxide particles at 300-850° C.    (Patent Document 1) Japanese Patent Application Laid-open No. 8-333117    (Patent Document 2) Japanese Patent Application Laid-open No. 2001-287997
The titanium oxide particles obtained by the method disclosed by Patent Document 1 have a large specific surface area and exhibit only poor dispersibility when used as a photocatalyst. In addition, handling of the titanium oxide particles is very difficult. The titanium oxide powder disclosed by Patent Document 2 has a high ratio of rutile to anatase of 20%.
These titanium oxide powders are mixtures of rutile-type titanium oxide and anatase-type titanium oxide. Even though their specific surface area is small and dispersibility is excellent, their ratio of rutile to anatase is comparatively high. Titanium oxide with a smaller ratio of rutile to anatase has a large specific surface area and exhibits only poor dispersibility. Therefore, titanium oxide powder exhibiting higher photocatalyst activity and excellent dispersibility has been demanded.
An object of the present invention is therefore to provide titanium oxide powder having a specific surface area larger than the specific surface area of the titanium oxide powder described in Patent Document 2 and having a ratio of rutile to anatase smaller than the ratio of rutile to anatase of the titanium oxide powder described in Patent Document 2, and a method for producing the titanium oxide powder.